Humidification of respiratory gases

ABSTRACT

A humidifier for a respiratory apparatus for delivering a humidified flow of breathable gas to a patient includes a humidifier chamber configured to store a supply of water to humidify the flow of breathable gas. The humidifier chamber includes a first heating element configured to heat the supply of water. The humidifier also includes a relative humidity sensor to detect a relative humidity of ambient air and generate signals indicative of the ambient relative humidity; a first temperature sensor to detect a temperature of ambient air and generate signals indicative of the ambient temperature; and a controller configured to determine an absolute humidity of the ambient air from the signals generated by the relative humidity sensor and the first temperature sensor and to control the first heating element to provide a predetermined relative humidity to the flow of breathable gas. A method of humidifying a flow of breathable gas to be provided to a patient includes determining an absolute humidity of ambient air used to form the flow of breathable gas; and controlling a temperature of a supply of water that humidifies the flow of breathable gas to provide a predetermined absolute humidity corresponding to a predetermined temperature and a predetermined relative humidity of the flow to be delivered to the patient.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/397,850, filed Mar. 4, 2009, now allowed, which claims priority toU.S. Applications 61/034,318, filed Mar. 6, 2008, 61/042,112, filed Apr.3, 2008, and 61/084,366, filed Jul. 29, 2008, the entire contents ofeach of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to systems and methods to control thehumidity of breathable gases used in all forms of respiratory apparatusventilation systems including invasive and non-invasive ventilation,Continuous Positive Airway Pressure (CPAP), Bi-Level therapy andtreatment for sleep disordered breathing (SDB) conditions such asObstructive Sleep Apnea (OSA), and for various other respiratorydisorders and diseases.

BACKGROUND OF THE INVENTION

Respiratory apparatuses commonly have the ability to alter the humidityof the breathable gas in order to reduce drying of the patient's airwayand consequent patient discomfort and associated complications. The useof a humidifier placed between the flow generator and the patient maskproduces humidified gas that minimizes drying of the nasal mucosa andincreases patient airway comfort. In addition in cooler climates, warmair applied generally to the face area in and about the mask is morecomfortable than cold air.

Many humidifier types are available, although the most convenient formis one that is either integrated with or configured to be coupled to therelevant respiratory apparatus. While passive humidifiers can providesome relief, generally a heated humidifier is required to providesufficient humidity and temperature to the air so that the patient willbe comfortable. Humidifiers typically comprise a water tub having acapacity of several hundred milliliters, a heating element for heatingthe water in the tub, a control to enable the level of humidification tobe varied, a gas inlet to receive gas from the flow generator, and a gasoutlet adapted to be connected to a patient conduit that delivers thehumidified gas to the patient's mask.

Typically, the heating element is incorporated in a heater plate whichsits under, and is in thermal contact with, the water tub.

The humidified air may cool on its path along the conduit from thehumidifier to the patient, leading to the phenomenon of “rain-out”, orcondensation, forming on the inside of the conduit. To counter this, itis known to additionally heat the gas being supplied to the patient bymeans of a heated wire circuit inserted into the patient conduit whichsupplies the humidified gas from the humidifier to the patient's mask.Such a system is illustrated in Mosby's Respiratory Care Equipment(7^(th) edition) at page 97. Alternatively the heating wire circuit maybe located in the wall of the patient conduit. Such a system isdescribed in U.S. Pat. No. 6,918,389.

In a hospital environment, the ambient temperature of the atmospherewithin the hospital is controlled by air conditioning to be generallyconstant at about, for example 23° C. The required temperature for thehumidified gases supplied by the respiratory apparatus may thus becontrolled within set temperature parameters. The controlled temperatureparameters ensure that the humidified gases are held at a temperatureabove their dew point to prevent condensation within the breathingcircuit.

Humidifiers are often used in a home care environment for use such as inthe treatment of breathing and sleep apnea disorders. Humidificationsystems used with CPAP devices for home use have been limited due topricing constraints, and by the need to maintain the systems small andlightweight, with a comfortable hose and mask, and a low complexity foruntrained users. In systems used in clinics or hospitals, suchconstraints are generally not an issue and temperature and humiditysensors may be located in the airpath and close to the patient's nose toprovide direct feedback to control systems, thus ensuring goodperformance. The cost, size, weight and discomfort of these systems havenot been suited to home use. Home users have therefore relied onexperience obtained through trial and error to achieve acceptableperformance.

In the home care environment, the range of ambient and gas temperaturesmay well exceed that of the hospital environment. In the home careenvironment temperatures as low as 10° C. may be present overnight andtemperatures over 20° C. may exist during the day. These temperaturevariations cause the commonly employed control techniques describedabove to suffer disadvantages. With the types of humidifiers described,condensation (or rain out) in the breathing conduit will exist, at leastto some degree. The degree of condensation is strongly dependent on theambient temperature, being much greater for greater differences betweenthe ambient temperature and the gas temperature. The formation of largequantities of water in the breathing tubing causes considerableinconvenience to the patient, may accelerate cooling of the gas, mayeventually occlude the tubing, create a gurgling sound in the tubing, ormay be expelled into the patient. Also, the patient may experiencediscomfort when breathing gases which are delivered at temperatureswidely divergent from that of the ambient temperature. Excessivecondensation also results in inefficient usage of the water in thehumidifying chamber of the humidifier.

An attempt to solve the problems associated with respiratory systems forhome use has involved monitoring ambient temperature and air flow rateas inputs to a control algorithm which predicts corrective heater inputto track the user's original setting. This approach, however, stillrelies on the user to determine an adequate setting for each usecondition.

SUMMARY

One aspect is a respiratory apparatus that resolves patient complaintsregarding inadequate warmth of the breathable gas delivered to thepatient interface, symptoms of nasal dryness, and/or excessivecondensation in the air delivery hose.

Another aspect is a respiratory apparatus that permits a patient toselect the temperature and/or relative humidity and/or absolute humidityof the breathable gas delivered to the patient interface. In analternative and/or additional aspect, an absolute humidity at the outletof a humidifier is controlled to regulate to a predetermined relativehumidity delivered to the patient.

A further aspect is a respiratory apparatus that provides a humidifiedflow to a patient interface at a predetermined temperature and/orhumidity while taking into account changing ambient temperature and/orhumidity.

A still further aspect is a respiratory apparatus that provides ahumidified flow of breathable gas to a patient interface at apredetermined temperature and/or humidity while taking into accountchanges in the rate of the flow of humidified flow of breathable gas.

Still another aspect relates to a respiratory apparatus comprising aflow generator and a humidifier that are connectable together to permitcommunication between the flow generator and the humidifier and/or toindicate connection and/or removal.

A further aspect relates to a respiratory apparatus that comprises ahumidifier and a heated air delivery tube, hose or conduit. A duty cycleof a heating element of the humidifier and a duty cycle of the heatedtube may be controlled so that a combined duty cycle does not exceed100%, and/or so that the humidifier heating element and the heated tubedo not receive power simultaneously. In an alternative and/or additionalaspect, the heating element of the humidifier and/or the heated tuberegulate a temperature rather than apply a fixed duty ratio. In afurther alternative and/or additional aspect, a temperature of thehumidified flow of breathable gas in the air delivery tube is measureddownstream of the humidifier to regulate to a predetermined relativehumidity delivered to the patient.

Another aspect relates to a flow generator that detects the connectionof a tube, for example a heated tube, and/or a size of a connected tube,and/or the disconnection of a tube from a humidifier.

Yet another aspect relates to a flow generator that includes constants,such as control parameters, for example stored in a table, that may betrilinearly interpolated to control the humidifier and/or the heatedtube.

A further aspect relates to a respiratory apparatus, and a controlthereof, including a humidifier and a non-heated tube connectable to thehumidifier.

Still another aspect relates to a humidifier control that convertspotential measured across, for example, a thermistor, to a temperature.

A further aspect relates to a respiratory apparatus comprising a flowgenerator and a humidifier that are connectable and may communicate dataand/or commands over a serial communications link.

According to a sample embodiment, a humidifier for a respiratoryapparatus for delivering a humidified flow of breathable gas to apatient comprises a humidifier chamber configured to store a supply ofwater to humidify the flow of breathable gas, the humidifier chambercomprising a first heating element configured to heat the supply ofwater; a relative humidity sensor to detect a relative humidity ofambient air and generate signals indicative of the ambient relativehumidity; a first temperature sensor to detect a temperature of ambientair and generate signals indicative of the ambient temperature; and acontroller configured to determine an absolute humidity of the ambientair from the signals generated by the relative humidity sensor and thefirst temperature sensor and to control the first heating element toprovide a predetermined absolute humidity to the flow of breathable gas.

According to another sample embodiment, a humidifier for a respiratoryapparatus for delivering a humidified flow of breathable gas to apatient comprises a humidifier chamber configured to store a supply ofwater to humidify the flow of breathable gas, the humidifier chambercomprising a first heating element configured to heat the supply ofwater; an absolute humidity sensor to detect an absolute humidity of thehumidified flow and generate signals indicative of the absolutehumidity; and a controller configured to receive the signals from theabsolute humidity sensor and control the first heating element toprovide a predetermined absolute humidity to the flow of breathable gas.

According to still another sample embodiment, a humidifier for arespiratory apparatus for delivering a humidified flow of breathable gasto a patient comprises a humidifier chamber configured to store a supplyof water to humidify the flow of breathable gas, the humidifier chambercomprising a first heating element configured to heat the supply ofwater; a relative humidity sensor to detect a relative humidity ofambient air and generate signals indicative of the ambient relativehumidity; a first temperature sensor to detect a temperature of ambientair and generate signals indicative of the ambient temperature; and acontroller configured to determine an absolute humidity of the ambientair from the signals generated by the relative humidity sensor and thefirst temperature sensor and to control the first heating element toprovide a predetermined absolute humidity, a predetermined temperature,and/or a predetermined relative humidity to the flow of breathable gas.

According to yet another sample embodiment, a humidifier for arespiratory apparatus for delivering a humidified flow of breathable gasto a patient comprises a humidifier chamber configured to store a supplyof water to humidify the flow of breathable gas, the humidifier chambercomprising a first heating element configured to heat the supply ofwater; an absolute humidity sensor to detect an absolute humidity ofambient air and generate signals indicative of the ambient absolutehumidity; a first temperature sensor to detect a temperature of ambientair and generate signals indicative of the ambient temperature; and acontroller configured to control the first heating element to provide apredetermined absolute humidity, a predetermined temperature, and/or apredetermined relative humidity to the flow of breathable gas.

According to a further sample embodiment, a respiratory apparatus forproviding a humidified flow of breathable gas to a patient comprises aflow generator to generate a flow of breathable gas and a humidifier asdiscussed above.

According to a still further sample embodiment, a method of humidifyinga flow of breathable gas to be provided to a patient comprisesdetermining an absolute humidity of ambient air used to form the flow ofbreathable gas; and controlling a temperature of a supply of water thathumidifies the flow of breathable gas to provide a predeterminedabsolute humidity corresponding to a predetermined temperature and apredetermined relative humidity of the flow to be delivered to thepatient.

According to another sample embodiment, a humidifier for a respiratoryapparatus for delivering a humidified flow of breathable gas to apatient comprises a humidifier chamber configured to store a supply ofwater to humidify the flow of breathable gas. The humidifier furthercomprises an inlet configured to receive the flow of breathable gas, afirst heating element configured to heat the supply of water, and anoutlet configured to deliver the humidified flow of breathable gas to aconduit. A controller is configured to control power supplied to theheating element to provide a predetermined absolute humidity to thehumidified flow of breathable gas. The controller continuously adjuststhe power supplied to the first heating element in response to changesin ambient conditions and/or the flow of breathable gas to continuouslyprovide the predetermined absolute humidity.

According to still another sample embodiment, a method of humidifying aflow of breathable gas to be provided to a patient comprises determiningan absolute humidity of ambient air used to form the flow of breathablegas; and controlling a temperature of a supply of water that humidifiesthe flow of breathable gas to provide a predetermined absolute humidityto the humidified flow. Controlling the temperature of the supply ofwater comprises adjusting the temperature of the water supply inresponse to a change in ambient air temperature, ambient air relativehumidity, ambient air absolute humidity, and/or the flow of breathablegas to continuously provide the predetermined absolute humidity.

According to a still further sample embodiment, a method of humidifyinga flow of breathable gas to be provided to a patient comprises detectinga temperature of the humidified flow at an end of a delivery hoseconfigured to be connected to a patient interface; generating signalsindicative of the temperature of the humidified flow at the end of thedelivery hose; and controlling a delivery hose heating element inresponse to the signals.

According to yet another sample embodiment, a humidifier for arespiratory apparatus for delivering a humidified flow of breathable gasto a patient comprises a humidifier chamber configured to store a supplyof water to humidify the flow of breathable gas, the humidifier chambercomprising a first heating element configured to heat the supply ofwater; an absolute humidity sensor to detect an absolute humidity ofambient air and generate signals indicative of the absolute humidity;and a controller configured to receive the signals from the absolutehumidity sensor and control the first heating element to provide apredetermined absolute humidity to the flow of breathable gas. Thepredetermined absolute humidity corresponds to a predeterminedtemperature and predetermined relative humidity.

BRIEF DESCRIPTION OF THE DRAWINGS

Sample embodiments will now be described with reference to theaccompanying drawings, in which:

FIG. 1 schematically depicts of a flow generator and humidifieraccording to a sample embodiment;

FIG. 2 schematically depicts the flow generator of FIG. 1;

FIG. 3 schematically depicts the humidifier of FIG. 1;

FIG. 4 schematically depicts an air delivery hose and patient interfaceaccording to a sample embodiment;

FIG. 5 schematically depicts the air delivery hose of FIG. 4 at anopposite end of the air delivery hose and an electrical connectorconnectable thereto;

FIG. 6 schematically depicts a respiratory apparatus according to asample embodiment;

FIG. 7 schematically depicts a relationship between absolute humidity ofambient air and temperature of ambient air at saturation with watervapor;

FIG. 8 schematically depicts a relationship between the temperature ofwater in a humidifier tub and ambient air temperature where there is nochange in ambient absolute humidity according to one example;

FIG. 9 schematically depicts a relationship between the temperature ofwater in a humidifier tub and the temperature of an air flow deliveredto a patient interface and ambient temperature where there is no changein ambient absolute humidity according to a comparative example;

FIG. 10 schematically depicts a change in water temperature in responseto a change in delivered gas temperature according to another example;

FIG. 11 schematically depicts a change in water temperature in responseto a change in ambient humidity where there is no change in ambienttemperature according to another example;

FIG. 12 schematically depicts a change in water temperature in responseto a change in the average gas flow rate through the humidifieraccording to one example;

FIG. 13 schematically depicts a humidifier according to another sampleembodiment;

FIG. 14 schematically depicts compensation of an example setting of thehumidifier by controlling the heating element of the humidifier inresponses to changes in ambient conditions and/or mean flow rate duringuse of the respiratory apparatus according to a sample embodiment;

FIG. 15 schematically depicts a control of the respiratory apparatusaccording to a sample embodiment;

FIG. 16 schematically depicts a control of the respiratory apparatusaccording to another sample embodiment;

FIG. 17 schematically depicts a control of the respiratory apparatusaccording to another sample embodiment; and

FIG. 18 schematically depicts a control of the respiratory apparatusaccording to another sample embodiment.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Humidification Theory

Humidity refers to the quantity of water vapor present in air. Humidityis commonly measured in two ways: absolute humidity (AH) and relativehumidity (RH). Absolute humidity is the actual content of water recordedin terms of weight per volume. Absolute humidity is usually measured ingrams per cubic meter (g/cm³) or milligrams per liter (mg/L).

Relative humidity is the percentage of the actual water vapor content ofa gas compared to its capacity to carry water at any given temperature.The capacity of air to hold water vapor increases as the temperature ofthe air increases. For air with a stable absolute humidity, the relativehumidity will decrease as the temperature of the air is increased.Conversely, for air saturated with water (i.e. 100% RH), if thetemperature is reduced, excess water will condense out of the air.

Air breathed by humans is heated and humidified by the airways to atemperature of 37° C. and 100% RH. At this temperature, the absolutehumidity is about 44 mg/L.

Humidification for CPAP

ISO 8185 requires that a medical humidifier be capable of supplying 10mg/L AH at a minimum, and 33 mg/L at a minimum if the patient's upperairway is bypassed. These minimum requirements are calculated with theinput of dry air. These minimum requirements are also only suitable forshort term use. These minimum requirements are often insufficient tominimize symptoms of nasal and upper airways dryness. Under normaloperating conditions, the patient or clinician should be able to set thetemperature of the air delivered to the patient interface from ambientto about 37° C. If no alarm system or indicator is provided to therespiratory apparatus, in accordance with ISO 8185, sections 51.61-51.8,under normal and single fault conditions, the temperature of the airdelivered to the patient interface should not exceed about 41° C.

For CPAP, the upper level of 44 mg/L may not be appropriate because thepatient's upper airway is not bypassed. On the other hand, the lowerlevel of 10 mg/L may be too low for CPAP, in particular for patientswith mouth leak.

Although no studies have been conducted to determine the minimum levelof humidification required for CPAP, Wiest et al. (Sleep, Vol. 24, No.4, pp. 435-440, 2001) found for patients in North America and Europe,the mean absolute humidity of 10 mg/L is too low when CPAP treatment isused without a humidifier system. The study tested two humidifiers whichboth supplied an absolute humidity of at least 23 mg/L. Wiest et al.concluded that the requirement for CPAP is above the 10 mg/L AH of ISO8185, but probably below the 23 mg/L AH used in the study. Applicantshave determined that an absolute humidity of about 20-30 mg/L providessuitable patient comfort.

Humidifier and Flow Generator

Referring to FIG. 1, a respiratory apparatus 1 may comprise a flowgenerator 2 and a humidifier 4 that are configured to be connectable toeach other. Such a flow generator and humidifier combination isdisclosed in, for example, WO 2004/112873 A1, the entire contents ofwhich are incorporated herein by reference. The humidifier may also be ahumidifier as disclosed, for example, in U.S. Pat. No. 6,935,337, theentire contents of which are incorporated by reference herein.

The flow generator 2 may comprise an ON/OFF switch 6 and a display 8,e.g. an LCD, to display the operational state of the flow generator, andother parameters as described in more detail below. The flow generator 2may also comprise buttons 14 for controlling the operation of the flowgenerator 2, for example to select various programs stored in a memoryof a controller configured to control operation of the flow generator.The buttons 14 may also be used to set various parameters, e.g., flowrate, of the flow generator 2.

The humidifier 4 may comprise a control knob 10 for controlling power toa heating element (not shown) and setting a temperature at a patientinterface as will be discussed in more detail below. Alternatively, thecontrols for the humidifier 4 may be incorporated within the flowgenerator 2. The humidifier 4 may also comprise an outlet 12 that isconfigured for connection to an air delivery hose or conduit fordelivering a humidified flow of breathable gas to a patient through apatient interface.

Referring to FIG. 2, the flow generator 2 may be formed of, for example,rigid plastics material molded in two parts, a top case 16 and a bottomcase 18. The top case 16 and the bottom case 18 define an engagementface 20 of the flow generator 2 that is configured to engage thehumidifier 4 when the humidifier 4 is connected to the flow generator 2.The engagement face 20 includes a pair of slots 22 configured to beengaged by corresponding tongues (not shown) provided on the humidifier4 by which the flow generator 2 and the humidifier 4 are connectedtogether. An electrical connection 24 may be provided to provide powerto the humidifier 4 when the flow generator 2 and the humidifier 4 areconnected. The flow generator 2 may further comprise an outlet 26configured to deliver a flow of breathable gas to the humidifier 4 whenthe flow generator 2 and humidifier 4 are connected.

As shown in FIG. 3, the humidifier 4 may comprise a hinged lid 28. Thehumidifier 4 may also include a tub as disclosed, for example, in U.S.Patent Application Publication 2008/0302361 A1, the entire contents ofwhich are incorporated herein by reference. The humidifier 4 may alsoinclude a heating element controllable by the control knob 10. Such aheating element is disclosed, for example, in WO 2008/148154 A1, theentire contents of which are incorporated herein by reference. Thehumidifier may also be heated, for example, as in WO 2004/112873 A1.

Although the flow generator and humidifier have been disclosed asseparate units connectable together to present an integrated unit, itshould be appreciated that the flow generator and humidifier may beprovided as separate elements that are not connectable together topresent an integrated appearance, for example as disclosed in U.S. Pat.No. 6,338,473, the entire contents of which are incorporated herein byreference.

Air Delivery Hose

Referring to FIG. 4, an air delivery conduit, or hose, 30 is connectedto a patient interface 32, e.g. a mask, to deliver the humidified flowof breathable gas from the humidifier outlet to a patient. It should beappreciated that the patient interface 32 may be a nasal mask, a fullface mask, nasal cannulae, pillows or prongs, or a combination of acushion configured to surround the patient's mouth and nasal prongs orpillows.

The air delivery hose 30 may be a heated tube, for example as disclosedin U.S. Patent Application Publication 2008/0105257 A1, the entirecontents of which are incorporated herein by reference. The air deliveryhose 30 may be formed by a tube 30 a formed of, for example,thermoplastic elastomer (TPE), and a helical rib 30 b formed of, forexample, very low density polyethylene. Wires 30 c, 30 d, 30 e aresupported by the helical rib 30 b so as to be in contact with the outersurface of the tube 30 a. The wires 30 c, 30 d, 30 e may be used to heatthe tube 30 a and to carry signals to and from a controller in flowgenerator 2 and/or the humidifier 4. It should be appreciated that theair delivery hose 30 may comprise two wires, and the signals may bemultiplexed over the two wires. It should also be appreciated that theair delivery hose 30 may include a heating element, for example in theform of a heating strip or wire, as disclosed for example in WO2009/015410 A1, the entire contents of which are incorporated herein byreference.

The air delivery hose 30 comprises a connector, or cuff 34 that isconfigured to connect the air delivery hose 30 to the patient interface32. The patient interface cuff 34 may comprise a temperature sensor, forexample a thermistor as disclosed in U.S. Patent Application Publication2008/0105257 A1, the entire contents being incorporated herein byreference, to sense a temperature of the humidified flow of breathablegas delivered to the patient interface 32.

Referring to FIG. 5, the air delivery hose 30 comprises a connector, orcuff 36 that is configured to be connected to the outlet 12 of thehumidifier. The humidifier cuff 36 comprises an end 36 a configured tobe connected to the outlet 12 and a grip portion 36 b to provide abetter grip for connecting and disconnecting the air delivery hose 30 toand from the outlet 12.

The humidifier cuff 36 may be connected to a controller of thehumidifier 4 by an electrical connector 38. The electrical connector 38provides power to the wires 30 c, 30 d, 30 e of the air delivery hose 30to heat the air delivery hose 30 along its length from the humidifier 4to the patient interface 32.

Respiratory System

Referring to FIG. 6, a respiratory system according to a sampleembodiment of the invention may comprise the flow generator 2, thehumidifier 4, and the air delivery hose 30. A patient interface 32 maybe connected to the air delivery hose 30.

The flow generator 2 may comprise a controller 40. The flow generatorcontroller 40 may comprise, for example, a programmable logic controlleror an application specific integrated circuit (ASIC). The flow generator2 may further comprise a flow sensor 42 to sense the volume (e.g. L/min)of the flow of breathable gas generated by the flow generator 2 anddelivered to the input of the humidifier 4. It should be appreciatedthat the flow may be estimated from a speed of a motor of the flowgenerator rather than provided directly from a flow sensor.

The humidifier 4 may comprise a controller 44. The humidifier controller44 may be, for example, a programmable logic controller or ASIC. Itshould be appreciated that, in the case of a flow generator and ahumidifier being connectable to form an integrated unit, the flowgenerator controller and the humidifier controller may be a singlecontroller configured to control both devices. Alternatively, thecontroller 40 of the flow generator can include all of the functionalityof the controller 44 and when the humidifier is attached, thefunctionality relating to humidification can be accessed from thecontroller 40.

The humidifier 4 further comprises a heating element 46 configured toheat a supply of water stored in the humidifier 4. The heating element46 may be, for example, a plate that is provided below a tub of thehumidifier. It should also be appreciated that the heating element 46may comprise a heating element as disclosed, for example, in WO2009/015410 A1, the entire contents of which are incorporated herein byreference. A temperature sensor 48 may be provided to sense atemperature of the water heated by the heating element 46. It should beappreciated that the temperature of the water may be determined bysensing or measuring the temperature of the heating element 46, forexample by using a temperature sensor to directly sense the heatingelement temperature.

The humidifier 4 may further comprise an ambient temperature sensor 50to detect the temperature of the ambient air and a relative humiditysensor 52 to detect a relative humidity of the ambient air. Thehumidifier may also optionally comprise an ambient pressure sensor 53.It should be appreciated that the sensors 50, 52, 53 need not beprovided on the humidifier, but may be provided separately, for examplefrom a station that includes the sensors and is connectable to thehumidifier 4. It should also be appreciated that the sensors 50, 52, 53may be provided to the flow generator 2 instead of the humidifier 4, orthe ambient temperature, relative humidity, and ambient pressure may beprovided from a station to the flow generator 2 instead of thehumidifier 4. It should further be appreciated that the flow sensor 42may be provided to the humidifier 4 instead of, or in addition to, theflow generator 2. It should be even further appreciated that the ambienttemperature, relative humidity, and ambient pressure sensors 50, 52, 53may be replaced by an absolute humidity sensor configured to detect theabsolute humidity of the humidified flow, for example at the humidifieroutlet, and generate signals indicative of the absolute humidity.

The air delivery hose 30 includes a temperature sensor 54, for example athermistor, in the patient interface cuff 34. It should be appreciatedthat the temperature sensor 54 may be provided in the patient interface32 instead of the cuff 34. The temperature detected by the temperaturesensor 54 may be sent as a signal through the air delivery hose 30 tothe humidifier controller 44.

The system of FIG. 6 may be configured to allow the patient to selectand set a temperature of the humidified flow of breathable gas that isdelivered to the patient interface 32. For example, the system may beconfigured to allow the user to set the temperature of the humidifiedflow at the patient interface 32 using either the control knob 10 of thehumidifier 4 or the control buttons 14 of the flow generator 2. Forexample, the system may be configured to permit the patient or clinicianto select a temperature of the humidified flow at the patient interfaceof about 10°-37° C., for example about 26° C.-28° C. The system may beconfigured to prevent the patient, or clinician, from selecting and/orsetting a temperature that is below ambient temperature. The ambienttemperature may be displayed on the display 8 of the flow generator, ora message may be displayed that indicates to the patient or clinicianthat the selected temperature is invalid as it is below the ambienttemperature. Alternatively, the system may allow selection of anautomatic, or default, temperature setting of, for example, 27° C.

The system of FIG. 6 may also be configured to provide an absolutehumidity to the patient interface 32 that is between, for example, about10-44 mg/L. The relative humidity of the flow at the patient interfacemay be controlled to have less than 100% RH, for example about 70-90%RH, for example about 80% as a default setting. Keeping the relativehumidity of the flow in the air delivery hose 30 below 100% RH helps toprevent rain out in the air delivery hose between the humidifier 4 andthe patient interface 32. The system may also be configured to providean automatic, or default, relative humidity at the patient interface 32,for example 80%. The system may also be configured to allow the patientor clinician to set the relative humidity of the flow at the patientinterface 32. Although the relative humidity of the flow at the patientinterface 32 may be detected directly by a humidity sensor placed in thepatient interface, as humidity sensors are prone to misread ormalfunction in the presence of condensation, a more reliable approachmay be to detect the relative humidity and the temperature of theambient air, or the incoming gas flow, and calculate the absolutehumidity.

The system of FIG. 6 may compensate for a wide variation of ambienttemperature and humidity. The temperature of the flow at the patientinterface 32 may be detected directly, for example by the temperaturesensor 54. The relative humidity of the flow at the patient interfacemay be calculated from: 1) the water content of ambient air (from itsambient temperature and relative humidity); 2) the temperature of thewater in the humidifier tub (e.g. as detected by the temperature sensor48); and/or 3) the flow rate through the humidifier tub (e.g. asdetected by the flow sensor 42 of the flow generator). It should beappreciated that the relative humidity may also be detected directly,for example by a relative humidity sensor at the end of the tube 30, orin the patient interface 32.

The temperature of the flow at the patient interface 32 may becontrolled by controlling the power supplied to the air delivery hose30, e.g., by controlling the current to the wires of the hose 30. Therelative humidity of the flow at the patient interface 32 may becontrolled by the temperature of the water in the humidifier tub, takingthe ambient temperature, the ambient relative humidity, and flow rate asinput parameters.

Humidity Control

Referring to FIG. 7, the absolute humidity of saturation of ambient aircan be calculated from the psychrometric properties of water vapor. See,for example, Heat Transfer, Y. Cengel, McGraw-Hill, 1998 (pp. 958-59,table A9). See also, for example, Release on the IAPWS IndustrialFormulation 1997 for the Thermodynamic Properties of Water and Steam,The International Association for the Properties of Water and Steam,September, 1997, Erlangen, Germany. As shown in FIG. 7, the absolutehumidity is expressed in mg/L, i.e., mass of water vapor per volume ofair, where the conditions are ambient temperature and standard (sealevel) pressure, or ATPS. The absolute humidity AHa of the ambient airmay be defined by any equation or look-up table that corresponds to thepsychrometric properties of water vapor. For example, the followingquadratic equation:

AHa=RHa·(K ₁ −K ₂ ·Ta+K ₃ ·Ta ²)   (1),

wherein RHa is the relative humidity of the ambient air, Ta is thetemperature of the ambient air, and K₁, K₂, and K₃ are coefficients. Forexample, coefficients K1, K2, K3 may be empirically determined, such asby curve fitting to available data. K₁ may be equal to 7.264, K₂ may beequal to 0.09276, and K₃ may be equal to 0.02931, for example.

The target temperature Tm of the flow at the mask and the targetrelative humidity RHm at the mask similarly determine the absolutehumidity AHm at the mask, as defined by the following equation:

AHm=RHm·(K ₁ −K ₂ ·Tm+K ₃ ·Tm ²)   (2),

wherein K₁=7.264, K₂=0.09276, and K₃=0.02931, for example.

The difference ΔAH between the ambient absolute humidity AHa, asdetermined by equation (1), and the mask absolute humidity AHm, asdetermined by equation (2), is equal to the absolute humidity to beadded to the flow by the humidifier 4. Of course, if AHm<AHa, nohumidification is required. Given the flow rate F (L/min) through thehumidifier tub, the evaporation rate E of water may be determined usingan equation derived by characterizing the response of the humidifier.For example, in one embodiment the evaporation rate may be determinedfrom the flow rate and change in absolute humidity by the followingequation:

E (g/hr)=ΔAH (mg/L)·F (L/min)·(60 min/hr)·0.001 g/mg   (3).

As an example, for a CPAP treatment using the system of FIG. 6, apressure of 10 cm H₂O is to be supplied to treat a patient with OSA. At10 cm H₂O, the flow rate F is about 35 L/min, which is equivalent to themask vent flow at the prescribed pressure. At an ambient temperature Taof 20° C. and an ambient relative humidity RHa of 50%, the absolutehumidity AHa of the air entering the humidifier, from equation (1), isequal to 0.5·17.3=10.4 mg/L. Assuming the patient selects a masktemperature Tm of 25° C. and a relative humidity of 90% is selected, orautomatically set, the absolute humidity AHm at the mask, from equation(2) is equal to 0.9·23.3=20.9 mg/L. The absolute humidity ΔAH to beadded by the humidifier is equal to 20.9−10.4=10.5 mg/L. The evaporationrate E from the humidifier, from equation (3) is thus determined asE=(10.5 mg/L)·(35 L/min)·(60 min/hr)·(0.001 g/mg)=22 g/hr.

The evaporation rate of water is related to its vapor pressure, drivenby the temperature of the liquid water. Generally speaking, each 10° C.rise in water temperature almost doubles the saturation vapor pressure.See, for example, Heat Transfer, Y. Cengel, McGraw-Hill, 1998 (pp.958-59, table A9). See also, for example, Release on the IAPWSIndustrial Formulation 1997 for the Thermodynamic Properties of Waterand Steam, The International Association for the Properties of Water andSteam, September, 1997, Erlangen, Germany. In addition, the ambient airwater content, i.e. the vapor pressure of water already in the ambientair, as determined from the ambient air temperature and ambient airrelative humidity, reduces the evaporation rate. The atmosphericpressure of the ambient air also effects the evaporation rate, but lessso than the rise in water temperature and the ambient air water content.Water vapor evaporates more rapidly at lower atmospheric pressure, e.g.at higher altitudes.

The temperature of the water in the humidifier tub may be subjected toclosed-loop control. Alternatively, the temperature of the heatingelement under the water maybe subjected to closed-loop control. Otherparameters may contribute to the setpoint for the closed-loop control.For example, the evaporation rate E is limited by saturation of watervapor in the humidifier tub. The saturation of water vapor in the tubdepends on the temperature of the air flowing into the humidifier fromthe flow generator. The flow generator may increase the temperature ofthe air flowing into the humidifier, e.g. from heat generated by theflow generator motor.

The theoretical relationship between the evaporation rate and thetemperature of the water also assumes that the water vapor in thehumidifier tub is efficiently removed from the tub. However, the patternof airflow through the tub may bypass some pockets where water vapor isgenerated. In addition, a stirring action from the airflow maydistribute heat evenly through the water in the tub.

The theoretical relationship also assumes that the evaporation rate islargely unaffected by the temperature of the air in the tub untilsaturation is reached. In practice, chilling the surface of the water,for example by a decrease in the ambient air temperature, may reduce theevaporation rate. Temperature gradients exist from heating the tubthrough the water and walls of the tub to the outside of the humidifier.These temperature gradients may contribute inconsistencies between thesensed temperature and the actual temperature at the surface of thewater. Even if a temperature sensor is not used, the temperaturegradients may contribute to inconsistencies between the temperature ofthe body of water and the temperature of the surface of the water. Theevaporation rate is related to the temperature at the water surface.

EXAMPLE 1 Adjustment to Change in Ambient Temperature with MaskTemperature Controlled

In this example, the system of FIG. 6 is set to deliver saturated air tothe mask at 30° C. The temperature may be set by the patient, orclinician, by use of the control buttons 14 of the flow generator 2and/or the control knob 10 of the humidifier 4. The absolute humidity ofthe ambient air is 10 mg/L, which does not change as the temperature ofthe ambient air changes, for example in the patient's bedroom, duringthe patient's sleep. As shown in Table 1, the temperature of the waterin the humidifier tub is adjusted to achieve 100% RH air at the patientinterface.

TABLE 1 Air Humidity Humidity Water Air absolute Air Temperature of airof air temperature temperature humidity relative Humidification of airdelivered delivered degC. degC. (mg/L) humidity output mg/L delivered(mg/L) % RH 64.9 15 10 80% 20.7 30 30.7 100% 64.9 16 10 75% 21.0 30 31.0100% 64.7 17 10 71% 20.9 30 30.9 100% 64.5 18 10 66% 20.9 30 30.9 100%64.3 19 10 62% 20.7 30 30.7 100% 64.3 20 10 58% 20.8 30 30.8 100% 64.221 10 55% 20.7 30 30.7 100% 64.3 22 10 52% 20.9 30 30.9 100% 64.3 23 1048% 20.9 30 30.9 100% 64.3 24 10 46% 20.9 30 30.9 100% 64.3 25 10 43%20.8 30 30.8 100% 64.3 26 10 41% 20.8 30 30.8 100% 64.3 27 10 38% 20.730 30.7 100% 64.4 28 10 36% 20.8 30 30.8 100% 64.5 29 10 34% 20.8 3030.8 100% 64.5 30 10 32% 20.7 30 30.7 100%

As shown in FIG. 8, the result of the closed-loop control is that thetemperature of the water in the tub must be held to about the samesetpoint, regardless of the ambient temperature of the air in the room.Thus, there is no need for the system to respond to changes in theambient air temperature if the temperature at the mask is regulated.

COMPARATIVE EXAMPLE 1 Adjustment to Change in Ambient Temperature withMask Temperature Uncontrolled

In this comparative example, the temperature of the air delivered to thepatient interface is not under a feedback control loop. Instead, thesystem is controlled so that the temperature of the water in thehumidifier tub was controlled to track the ambient air temperature, asshown in Table 2 and FIG. 9.

TABLE 2 Air Humidity Humidity Water Air absolute Air Temperature of airof air temperature temperature humidity relative Humidification of airdelivered delivered degC. degC. (mg/L) humidity output mg/L delivered(mg/L) % RH 38.5 15 10 80% 2.5 15 12.5 100% 40.2 16 10 75% 3.3 16 13.3100% 42 17 10 71% 4.1 17 14.1 100% 44 18 10 66% 5.1 18 15.1 100% 46 1910 62% 6.2 19 16.2 100% 47.8 20 10 58% 7.2 20 17.2 100% 49.5 21 10 55%8.3 21 18.3 100% 51.2 22 10 52% 9.4 22 19.4 100% 53 23 10 48% 10.6 2320.6 100% 54.8 24 10 46% 12.0 24 22.0 100% 56.5 25 10 43% 13.3 25 23.3100% 58 26 10 41% 14.5 26 24.5 100% 59.8 27 10 38% 16.1 27 26.1 100%61.5 28 10 36% 17.7 28 27.7 100% 63 29 10 34% 19.2 29 29.2 100% 64.5 3010 32% 20.7 30 30.7 100%

In this comparative example, although the relative humidity of the airdelivered to the patient interface is 100% RH for all temperatures, theabsolute humidity of the air delivered to the patient interface varieswidely, for example from 12.5 mg/L-30.7 mg/L. The temperature of the airflow delivered to the patient interface also varies according to theambient air temperature. The patient thus is unable to increase thetemperature of the air flow delivered to the patient interface.

EXAMPLE 2 Adjustment to Change in Set Temperature at Patient Interface

Referring to Table 3 and FIG. 10, in this example, the system of FIG. 6is controlled to deliver saturated air and the temperature at thepatient interface is changed.

TABLE 3 Air Humidity Humidity Water Air absolute Air Temperature of airof air temperature temperature humidity relative Humidification of airdelivered delivered degC. degC. (mg/L) humidity output mg/L delivered(mg/L) % RH 53 22.5 10 50% 10.6 23 20.6 100% 54.6 22.5 10 50% 11.8 2421.8 100% 56.5 22.5 10 50% 13.4 25 23.4 100% 58 22.5 10 50% 14.6 26 24.6100% 59.6 22.5 10 50% 16.1 27 26.1 100% 61.2 22.5 10 50% 17.6 28 27.6100% 62.8 22.5 10 50% 19.3 29 29.3 100% 64.2 22.5 10 50% 20.8 30 30.8100% 65.8 22.5 10 50% 22.6 31 32.6 100% 67.2 22.5 10 50% 24.2 32 34.2100% 68.7 22.5 10 50% 26.1 33 36.1 100% 70.2 22.5 10 50% 28.0 34 38.0100% 71.7 22.5 10 50% 30.0 35 40.0 100%

The ambient air is assumed to be 22.5° C., the absolute humidity is 10mg/L, and the relative humidity is 50%. The ambient air conditions areassumed not to change. The temperature of the water in the humidifiertub is adjusted to achieve 100% RH at the patient interface, as shown inTable 3. As the requested temperature at the patient interface isincreased, the temperature of the water in the humidifier tub isincreased to maintain saturation of the air delivered. As shown in FIG.10, the relationship between the temperature of the water in thehumidifier tub and the temperature of the air delivered is approximatelylinear, e.g. there is approximately an increase of 1.55° C. in the watertemperature for each 1° C. increase in the temperature of the airdelivered to the patient interface. The temperature at the mask may beautomatically and independently controlled by controlling the power tothe heated tube.

In this example, the temperature of the air delivered to the patientinterface may be selected by the patient, or clinician, through the useof, for example, the control buttons 14 of the flow generator 2. Thepatient may select an operation mode that permits adjustment of thetemperature of the air at the patient interface. The heating element ofthe humidifier is then automatically controlled to increase thetemperature of the water in the humidifier tub as the requestedtemperature of the air at the patient interface is increased, andcorrespondingly decrease the water temperature as the requested airtemperature decreases.

EXAMPLE 3 Adjustment to Change in Ambient Humidity

The system of FIG. 6 may also be configured to adjust for changes inambient humidity. For example, signals from the sensors 50, 52 may beprovided to the controller(s) 40 and/or 44 to periodically orcontinuously calculate the absolute humidity of the ambient air. Asshown in Table 4 and FIG. 11, the temperature of the ambient air ismaintained relatively constant, for example 22.5° C., but the absolutehumidity changes throughout the course of the patient's sleep cycle.

TABLE 4 Air Humidity Humidity Water Air absolute Air Temperature of airof air temperature temperature humidity relative Humidification of airdelivered delivered degC. degC. (mg/L) humidity output mg/L delivered(mg/L) % RH 68.5 22.5 4 20% 26.8 30 30.8 100% 68 22.5 5 25% 26.0 30 31.0100% 67.2 22.5 6 30% 24.9 30 30.9 100% 66.5 22.5 7 35% 23.9 30 30.9 100%65.8 22.5 8 40% 22.9 30 30.9 100% 65 22.5 9 45% 21.8 30 30.8 100% 64.222.5 10 50% 20.8 30 30.8 100% 63.5 22.5 11 55% 19.9 30 30.9 100% 62.822.5 12 60% 19.0 30 31.0 100% 62 22.5 13 65% 18.0 30 31.0 100% 61 22.514 70% 16.9 30 30.9 100% 60 22.5 15 75% 15.8 30 30.8 100% 59.2 22.5 1680% 14.9 30 30.9 100% 58.2 22.5 17 85% 13.8 30 30.8 100% 57.2 22.5 1890% 12.8 30 30.8 100% 56.2 22.5 19 95% 11.9 30 30.9 100% 55.2 22.5 20100%  11.0 30 31.0 100%

The temperature of the water in the humidifier tub is adjusted toachieve 100% RH at the patient interface. The temperature at the patientinterface is maintained constant, for example 30° C. The temperature ofthe water in the humidifier tub is thus decreased as the ambient airabsolute humidity, and relative humidity, increases. Control of thesystem in this manner permits the temperature of saturated air to bedelivered to the patient interface to be maintained relatively constant,as shown in Table 4.

EXAMPLE 4 Adjustment to Change in Air Flow Rate

Referring to Table 5 and FIG. 12, the ambient temperature and relativehumidity and the temperature of the air delivered to the patientinterface are constant.

TABLE 5 Air Humidity Humidity Water Air absolute Air Temperature of airof air temperature temperature humidity relative Humidification of airAirflow delivered delivered degC. degC. (mg/L) humidity output mg/Ldelivered L/min (mg/L) % RH 54.8 22.5 10 50% 21.0 30 20 31.0 100% 56.322.5 10 50% 21.0 30 22 31.0 100% 57.7 22.5 10 50% 21.0 30 24 31.0 100%59 22.5 10 50% 20.9 30 26 30.9 100% 60.3 22.5 10 50% 21.0 30 28 31.0100% 61.6 22.5 10 50% 21.1 30 30 31.1 100% 62.7 22.5 10 50% 21.0 30 3231.0 100% 63.8 22.5 10 50% 20.9 30 34 30.9 100% 64.9 22.5 10 50% 21.0 3036 31.0 100% 66 22.5 10 50% 21.0 30 38 31.0 100% 67 22.5 10 50% 21.0 3040 31.0 100% 68 22.5 10 50% 21.0 30 42 31.0 100% 68.9 22.5 10 50% 20.930 44 30.9 100% 69.9 22.5 10 50% 21.0 30 46 31.0 100% 70.8 22.5 10 50%21.0 30 48 31.0 100% 71.7 22.5 10 50% 21.0 30 50 31.0 100%

The flow rate through the humidifier is adjusted, for example throughthe action of ResMed's AUTOSET® control algorithm. The flow rate mayalso be adjusted, for example, in response to a leak at the patientinterface. As shown in Table 5 and FIG. 12, the temperature of the waterin the humidifier tub is increased as the air flow rate increases tomaintain saturation at the patient interface.

The respiratory system may be controlled according to each of Examples1-4 and combinations thereof. The data provided in Tables 1 and 3-5 andFIGS. 8 and 10-12 may be stored in a memory, for example incontroller(s) 40 and/or 44. The controllers 40, 44 may be programmed tolook up data from the stored information. The controllers 40, 44 mayalso be programmed to interpolate and/or extrapolate data from thestored information. The setpoint of the heating element that providesthe appropriate evaporation rate for each combination of ambienttemperature and humidity, flow and predetermined output humidity may bedetermined experimentally to characterize the design and then built intothe controller, for example stored as a table in a memory or as a set ofequations.

Although the relative humidity of the air delivered to the patientinterface in each of Examples 1-4 is described as 100%, it should beappreciated that the relative humidity of the air delivered to thepatient interface may be about 50%-100%, for example about 70%-90%, orabout 80% as another example, or any other value selected by the patientor clinician.

Humidifier Control

The humidifier 4 may provide a user selectable setting that will providean automatic delivery of a predetermined moisture content at the mask32. An example value for delivered air moisture content is determinedtaking into account the conditions which lead to unwanted condensationin the tube 30. For users with normal upper airways a desiredphysiological outcome is to condition the air to approximate normalinspiratory conditions at the nose. For example, the ambient air may be20° C. and 25% RH (4 mg/L AH). The air may be heated and humidified toconditions equivalent to about 20° C. at 80% RH (14 mg/L AH). A moisturecontent of 14 mg/L, which corresponds to the absolute humidity at 20° C.at 80% RH, may therefore be chosen to be the example value. Thehumidifier would be set to hold the output to 14 mg/L. The difference of10 mg/L would be added by the humidifier. It should be appreciated thatalthough this value may be chosen as the example value, and thehumidifier may be configured to include a user setting thatautomatically provides this value, the example value may be determined,or revised, on the basis of clinical advice and the humidifier may beconfigured, or reconfigured, to include a user setting thatautomatically provides a clinically determined moisture content. Forexample, the patient or clinician may select an absolute humidity fromabout 10 mg/L-25 mg/L, for example 20 mg/L which corresponds generallyto a relative humidity between 70%-80% at a temperature of approximately27°-28° C.

The actual temperature of delivered air in a CPAP system may be higherthan room ambient temperature, typically about 29° C., in the case of arespiratory apparatus provided with heated tubing. Therefore, the RHvalue at the nose would be less than 50% for the same absolute humidityvalue. In the case of a respiratory apparatus without heated tubing, thehumidified air cools in the tubing to one or two degrees above ambient.Without heated tubing, the air would be delivered at about 22° C. and70% RH (14 mg/L AH).

At the optimum setting, e.g. 10 mg/L, condensation in the breathing tubewill not occur unless the room temperature falls sufficiently to causethe temperature of the delivered air to fall below its dew point (beingapproximately 16° C. for air at 29° C. as typically delivered to themask with a CPAP apparatus operating in 22° C. ambient). Should the roomtemperature continue to drop, causing the delivered air temperature toalso drop, then the heater temperature is automatically reduced to lowerthe delivered moisture content below the optimum level in order to avoidcondensation, but still to target the optimum level as closely aspossible.

Referring to FIG. 13, a humidifier 4 according to another sampleembodiment of the invention comprises a control knob 10 including asetting indicator 10. The humidifier 4 comprises indicia 11 thatindicate a plurality of settings. An indicia 11 a may designate theautomatic setting that provides the default moisture content. Forexample, as shown in FIG. 13, the automatic setting indicia 11 a maycomprise, for example, a ▾. It should be appreciated that any otherindicia may be used, for example the indicia 11 a may include the words“optimum” or “automatic.” The remaining indicia 11 may comprise numbers,for example 1-4 and 6-9, that permit the user to increase and decreasethe delivered humidity. The indicia may also be configured to showvalues for the relative or absolute humidity and temperature, forexample as percentage RH. To select the default setting, the user alignsthe setting indicator 10 a of the control knob 10 with the automaticsetting indicia 11 a. To adjust the humidity setting, the user alignsthe setting indicator 10 a with any of the other indicia 11, or at anyposition between any of the other indicia 11. For example, to lower thehumidity setting, the user may align the setting indicator 10 a with anyone of the numbers 1-4, or any setting therebetween. Similarly, toincrease the humidity setting, the user may align the setting indicator10 a to any one of the numbers 6-9, or any setting therebetween. Itshould also be appreciated that the control may be other than a knob,for example the control may include a display, such as an LCD, todisplay the setting, and a button, or buttons, to permit selection ofthe setting, or to change the displayed setting.

As shown in FIG. 14, when the automatic default moisture content isselected, i.e. by aligning the setting indicator 10 a with the indicia11 a, the humidifier 4 may be controlled so that the heating element 46of the humidifier 4 is continuously adjusted to maintain the moisturecontent of the flow of breathable gas at the predetermined defaultlevel, e.g. 14 mg/L. As discussed in more detail below, the heatingelement 46 is continuously adjusted to maintain the moisture content ofthe flow to a value as close to the default level as possible whilestill preventing condensation, or rain-out, in the tube 30.

In response to changing room conditions during the user's sleep session,e.g. ambient temperature, ambient relative humidity, and/or ambientpressure, and/or in response to changes in the flow, the heating element46 is controlled to maintain the default moisture content, e.g. 14 mg/L.For example, at the start of the patient's sleep session (condition 1),the room conditions may be at a first temperature, a first relativehumidity and a first pressure. The flow generator may produce a firstflow Q1 at the start of the patient's sleep session. The heating element46 of the humidifier 4 is controlled so that the default moisturecontent, e.g. 14 mg/L, is provided when the patient selects theautomatic setting by aligning the setting indicator 10 a with theindicia 11 a.

Although condition 1 is described above as corresponding to the start ofthe patient's sleep session, it should be appreciated that condition 1may correspond to a time from start up of the respiratory system, forexample a warm-up time that takes into account the effect of thedelivered air temperature above the ambient temperature.

During the course of the patient's sleep session, room conditions,including the ambient temperature, ambient relative humidity, and/orpressure, may change to a second condition (condition 2). The flow Q2generated by the flow generator may also change during the course of thepatient's sleep session. The heating element 46 of the humidifier 4 iscontrolled so that the moisture content of the flow is the defaultcontent, e.g. 14 mg/L at condition 2, regardless of the change in theroom conditions.

Similarly, if the patient chooses a different setting at start up(condition 1), for example by aligning the setting indicator 10 a withthe indicia “9” (to increase the moisture content from default) or theindicia “1” (to decrease the moisture content from default), the heatingelement 46 is controlled so that the moisture content that is deliveredto the mask is the same at condition 2 as is delivered at condition 1.The full range of moisture content settings, which is centered about thedefault setting, is continuously and automatically re-scaled in responseto the monitored values of the ambient temperature, ambient relativehumidity, ambient pressure, and delivered flow so that the selectedsetting is always calibrated to deliver the selected moisture content.

Humidity Control First Embodiment

Referring to FIG. 15, a control system and process for the respiratoryapparatus is illustrated. At S1, a temperature Tm of the flow to bedelivered to the mask is determined. At S2, a relative humidity RHm ofthe flow to be delivered to the mask is determined. It should beappreciated that the user may set the temperature Tm and the relativehumidity RHm, for example by using the buttons 14 on the flow generator2. Alternatively, the user may select a moisture content, i.e. absolutehumidity, of the flow to be delivered to the mask by adjusting thecontrol knob 10 of the humidifier. For example, the user may align thesetting indicator 10 a to the default setting indicia 11 a. The defaultmoisture content may be a nominal moisture content, e.g. 14 mg/L, or aclinically determined moisture content. The user may also select amoisture content other than the default by aligning the settingindicator 10 a with another of the indicia 11. In the case where thehumidifier is integrally connected to the flow generator, therespiratory apparatus may be configured to permit the user to select themoisture content using either the buttons 14 of the flow generator 2 orthe control knob 10 of the humidifier 4. In the case where the userselects a moisture content, the temperature Tm and the relative humidityRHm to be delivered to the mask correspond to the selected moisturecontent setting.

The heating element of the air delivery tube 30 is controlled to providethe predetermined temperature Tm to the flow delivered to the mask. Thetemperature sensor 54 at the end of the air delivery hose 30 senses theactual temperature of the flow at the end of the air delivery hose 30.The difference ΔTm between the predetermined temperature Tm and thesensed temperature is determined in S11 by the controller(s) 40, 44 andthe controller(s) 40, 44 adjusts the power to the heating element of theair delivery tube 30 until the difference between the predeterminedtemperature and the sensed temperature is substantially zero.

At S3, the temperature sensed by the sensor 54 and the predeterminedrelative humidity RHm to be delivered to the mask are inserted intoequation (2) to provide the absolute humidity AHm, i.e. moisturecontent, to be delivered to the mask at S4. At S5, the ambienttemperature Ta from the sensor 50 and the ambient relative humidity RHafrom the sensor 5 are inserted into equation (1) to provide the ambientabsolute humidity AHa at S6. At S7, the difference ΔAH between theabsolute humidity AHm to be delivered to the mask and the ambientabsolute humidity AHa is determined. The difference ΔAH is the absolutehumidity that the humidifier 4 must add to the flow in order to deliverthe selected moisture content.

At S8, the flow rate F as sensed by the flow sensor 42, or estimated, isinserted into equation (3) along with the difference ΔAH to determinethe required evaporation rate E from the supply of water in thehumidifier. At S9, the required water temperature, or equivalenttemperature of the humidifier heating element 46, to produce theevaporation rate E is determined, for example by the closed-loop controldiscussed above.

At S10, the difference ΔT between the water temperature as sensed by thesensor 48 and the required water temperature determined at S9 iscalculated. The controller(s) 40, 44 controls the heating element 46 ofthe humidifier 4 until the difference between the required watertemperature and the sensed water temperature is substantially zero.Alternatively, the heating element 46 is controlled until the differencebetween the required heating element temperature and the sensed heatingelement temperature is substantially zero.

Humidity Control Second Embodiment

Referring to FIG. 16, a control system and process for the respiratoryapparatus according to another sample embodiment is illustrated. Duringuse of the respiratory system, the flow rate may change, for examplethrough the action of the AUTOSET® control algorithm, which may providea relatively slow change in the flow rate, or due to the development ofleaks, either around the mask cushion or mouth leak from the use of anasal mask, which may provide a relatively fast change in the flow rate.If the flow rate changes rapidly, the control of the heated tubing orhose and/or humidifier may not respond rapidly enough to preventcondensation in the tubing as the response is relatively slow because ittakes a relatively longer time for the humidifier to change thetemperature of the supply of water.

As shown in FIG. 16, a change, or difference, ΔF in the flow rate sensedby the flow rate sensor 42, or for example estimated from the blowerspeed, is determined at S12. The difference ΔF may be determined bycomparing the sensed, or estimated, flow rate at periodic intervals. AtS13, the difference ΔF is compared to a predetermined differenceΔF_(ptd). If the difference ΔF between periodic flow rates exceeds apredetermined amount ΔF_(ptd), the process proceeds to S14 and thetemperature Tm of the gas delivered to the patient interface isadjusted, for example by controlling the heated tubing 30 using thecontroller(s) 40, 44. If the difference ΔF does not exceed thepredetermined amount ΔF_(ptd), the process proceeds similar to thatdescribed above with respect to the first embodiment and the requiredevaporation rate is calculated in S8 from the absolute humiditydifference ΔAH and the detected, or estimated, flow rate.

If the difference ΔF is negative, i.e. the change in the flow rate is adecrease, the temperature Tm is increased in S14. The temperature Tm maybe increased in S14 sufficiently to keep the temperature Tm of the airdelivered to the patient interface above the saturation point. Thedecrease in flow rate also results in a decrease of the watertemperature or the heating element temperature in S9, a calculation ofthe difference ΔT in S10, and control of the heating element by thecontroller(s) 40, 44 to reduce the temperature setpoint of thehumidifier. As the temperature of the water supply of the humidifier isreduced, there is a margin for the absolute humidity AHm to overshootwithout reaching the saturation point.

The difference ΔTm between the temperature sensed by the temperaturesensor 54 and the adjusted temperature Tm is determined in S11 and theheated tubing 30 is controlled until the difference ΔTm is substantiallyzero. Over a predetermined time period, the adjusted temperature Tm inS12 is gradually reduced until the temperature of the water supply inthe humidifier is reduced to the new setpoint of the humidifier.

If the flow rate difference ΔF determined in S11 is positive, i.e. thechange in flow rate is an increase, and greater than the predetermineddifference ΔF_(ptd), the adjustment in S14 may be a reduction of thetemperature Tm to keep the absolute humidity AHm close to saturation.However, the patient may find the reduction of the temperature Tm to beuncomfortable. In that case, the controller(s) 40, 44 may be configuredto disregard a flow rate difference ΔF that indicates an increase in theflow rate.

The humidifier and respiratory apparatus discussed herein with respectto the sample embodiments provide inexperienced or new users of heatedhumidifiers with an automatic, or default, setting that is designed toprovide the default moisture content in the delivered air (nominally 14mg/L) for any given use conditions. During the patient's sleep session,automatic compensation will be invoked to reduce the target value of thedefault moisture content, if necessary, to avoid condensation fromoccurring in the air tube.

Correct performance of the humidifiers according to the sampleembodiments disclosed herein does not require any user knowledge orintervention in order to properly set up and operate the device. Thisbenefits users who otherwise find it difficult to establish a suitablehumidifier setting. Correct performance is automatically maintainedduring the patient's sleep session, responding to changes to the factorsinfluencing the delivered air moisture content, and the potential forcondensation), these factors including ambient absolute humidity,ambient temperature, relative humidity and pressure, and delivered airflow rate.

The user is provided with additional settings to fine-tune theautomatic, or default, setting, if necessary, according to theirpreference. The full-scale range of available settings is continuouslyre-scaled to maintain the centered value to be calibrated at the defaultmoisture content, subject to the prevention of condensation in the airdelivery hose as discussed above. This means that, unlike prior arthumidifiers, the default setting and the available full-scale range ofsettings is always calibrated to actual ambient conditions. Climatedifferences in one region, e.g. cold wet climate, does not compromisethe available humidification performance or available settings inanother region as can be the case in devices with fixed heater settings.

For example, the user may determine that a setting lower than thedefault or automatic setting, such as “3” as marked by indicia 11, or asetting higher than the default or automatic setting, such as “7” asmarked by indicia 11, provides the most comfortable humidified flow. Theuser may therefore select the desired setting and the absolute humidityof the flow delivered to the patient interface will be the mostcomfortable, as determined by the patient, regardless of the ambientconditions and/or flow rate.

Humidifier Control Third Embodiment

Referring to FIG. 17, a control system and process for the respiratoryapparatus according to another sample embodiment is illustrated. Thesystem and process of the sample embodiment of FIG. 17 operates similarto the sample embodiment of FIG. 15 for S1-S8 and S11 as describedabove.

After calculating the required evaporation rate to deliver thepredetermined humidity at the predetermined temperature in S8, a heaterelement temperature threshold above which to apply a fixed duty ratio isdetermined in S15 and a duty ratio at which to drive the humidifier isdetermined in S16. After determining the heater element temperaturethreshold in S15, a determination is made in S17 whether the heatingelement temperature is above the threshold. If the heating elementtemperature is above the threshold (S17: Yes), a fixed duty ratio isapplied to the heating element in S21. If the heating elementtemperature is not above the threshold (S17: No), a duty ratio of 100%is applied to the heating element in S18.

A determination is made in S19 if the heating element temperature, assensed by the heating element temperature sensor 48, is above a safeoperating temperature. If the sensed heating element temperature isbelow the safe operating temperature (S19: No), the heating elementtemperature is checked again in S17 to determine if the heating elementtemperature is above the threshold. If the sensed heating elementtemperature is above the safe operating temperature (S19: Yes), the dutycycle of the heating element is set to 0%, i.e. the heating element isturned off, in S20.

A humidifier may be configured to operate using different types ofcontainers, or tubs, to contain the water supply. One such humidifier isdisclosed, for example, in U.S. Application 61/097,765, filed Sep. 17,2008, the entire contents of which are incorporated herein by reference.Two types of humidifier tub which may be used are a “reusable” tub with,for example, a stainless steel base, and a “disposable” tub with, forexample, an aluminium base. The thermal transfer properties differ inthe two bases. When the heating element is regulated to a constanttemperature, the two tubs may provide different humidity outputs.However, it is desirable that the humidity output be predictable nomatter which tub is fitted to the device. This is also preferable whenthere is means to detect which type of tub is fitted in the humidifier.

The humidity output may be correlated to the duty ratio at which theheating element is powered, rather than the temperature at which it isheld, as described above with reference to FIG. 17. This is because atconstant duty ratio the heater element is delivered power at a constantrate, and the main dissipation of that power in the system is throughevaporation of water from the tub. In practice, the “disposable” and the“reusable” tub have equivalent evaporation rates when operated at thesame duty ratio.

As also described with reference to FIG. 17, the sample embodimentapplies a constant duty ratio of power to the heating element, ratherthan varying the duty ratio to regulate the temperature of the hotplate. The heating element is a resistive load R, for example 9.6 ohm,to which a constant electrical potential V, for example 24V, that isswitched on and off with timing defined by the duty ratio. This isequivalent to driving the heating element with electrical power definedby P=V²/R, for example 60 W at 100% duty ratio.

The duty ratio may be determined in the same manner as the heatingelement temperature setpoint was determined in the sample embodiments ofFIGS. 15 and 16—through characterisation of the device for itsperformance considering three variables: the ambient absolute humidity,the temperature of gas delivered to the patient and the flow rate atwhich that gas is delivered.

Two disadvantages of constant duty ratio operation are also overcome bythis sample embodiment. The first disadvantage is that the body of waterin the humidifier takes much longer to warm up from a cold start. Thisis overcome by estimating the temperature threshold in S15 and drivingthe heating element with 100% duty ratio in S18 until the temperaturethreshold is reached, then switching to the constant, or fixed, dutylevel in S21 needed for the desired evaporation rate.

The second disadvantage is that the heating element could reach anexcessive temperature once the humidifier tub is empty of water, such aswhen it has all evaporated. This is overcome by applying a maximum safetemperature of operation in S19, above which the heating element isdisabled by setting the duty ratio to 0% in S20.

The sample embodiment of FIG. 17 may also be used to control thehumidifier in the absence of the heated tubing. In this case the patienthas direct control of the heating element regulated temperature throughthe user interface (e.g. the knob or dial 10 and/or the control buttons14) and can adjust it for their comfort, so patients using the reusabletub may tend to set slightly higher temperatures than patients using thedisposable tub. Without control of the duty ratio of the power suppliedto the heating element, the humidifier would deliver less humidity usingthe reusable tub than when using the disposable tub, and would offerless comfort benefit of humidification to the patient.

The sample embodiment of FIG. 17 also provides equivalent humidificationtherapy to all patients, maintaining simplicity.

Humidifier Control Fourth Embodiment

In addition to controlling the duty ratio of the heating element 46 ofthe humidifier, the controller(s) 40 and/or 44 may also be configured tocontrol the duty ratio of the heating element of the air delivery hoseor tube 30. This allows the humidifier to reduce the total capacity ofits power supply. The humidifier heating element and the heated tube mayshare power loading so that while either the humidifier heating elementor the heated tube can draw its full current, for example 2.5 A at 24V,instantaneously, they are never active simultaneously. The controller(s)40 and/or 44 calculate the duty ratio to assign to each of thehumidifier and the heated tube so that the combined duty cycle does notexceed 100%. The controller(s) 40 and/or 44 also synchronise the heatingcycles in the humidifier and the heated tube so that they do notoverlap. The controller(s) 40 and/or 44 may be configured to switch eachheating element on and off at timings according to the duty ratiosprovided by the flow generator so that only one device is on at a time.Such a power management control is disclosed in, for example, U.S.Application 61/095,714, filed Sep. 10, 2008, the entire contents ofwhich are incorporated herein by reference.

According to this sample embodiment, inputs include: 1) temperaturesetpoint for heated tubing, for example as set by the user interface ora climate control algorithm; 2) temperature sensed by the heated tubing,for example converted from a potential difference across a thermistor;3) type of heated tubing (15 mm or 19 mm for example); 4) temperaturesetpoint for humidifier, for example as set by the user interface or aclimate control algorithm; and 5) temperature sensed by the humidifier,for example as converted from a potential difference across thermistor.

Outputs of the sample embodiment include: 1) heating power to be appliedto humidifier, for example a duty ratio from 0 to 100%; and 2) heatingpower to be applied to heated tubing, for example a duty ratio from 0 to100%.

The control also includes the use of constants for the heated tubing,including: 1) a proportional factor Pf; 2) an integral factor If; and 3)a derivative factor Df. Similary, the control also includes the use ofconstants for the humidifier, including: 1) a proportional factor Pf; 2)an integral factor If; and 3) a derivative factor Df.

Internal variables include: 1) humidifier temperature sensed on previousreading, Told; 2) humidifier cumulative sum of temperature differences,sumTd; 3) heated tubing temperature sensed on previous reading, Told;and 4) heated tubing cumulative sum of temperature differences, sumTd.

The controller(s) 40 and/or 44 may comprise a simplifiedproportional-integral control function:

1. Calculate temperature difference Td=this temperature reading minusthe previous reading Told.

2. If the measured temperature is close to the setpoint (|Td| is lessthan 1/Pf),

-   -   a. then multiply Td times integral factor If and add the result        to the cumulative sum of temperature differences sumTd,    -   b. else reset sumTd to zero.

3. Calculate the duty ratio=Pf*Td+If*sumTd.

4. Trim the duty ratio to be between 0 and 1.

The duty ratios for each of the humidifier and heated tubing are thencompared.

1. If the sum of duty for humidifier and heated tubing exceeds 1.0, thenone or both duty ratios are reduced. For example, the heated tubing dutyratio is reduced to 0.5 and then the humidifier duty ratio will bereduced as far as necessary.

2. The two duty ratios are multiplied by 100 for output to thehumidifier controller as integer values from 0 to 100 (indicating 100%).

Humidifier Control Fifth Embodiment

According to another sample embodiment, the controller(s) 40 and/or 44may be configured to control the humidifier heating element and theheated tubing using inputs including: 1) air flow rate sensed by flowgenerator, for example averaged over one minute; 2) ambient relativehumidity, for example as determined or sensed by the humidifier; 3)ambient temperature, for example as sensed by the humidifier; 4) atemperature sensed by the heated tubing, for example in ° C., if aheated tube is connected; 5) a heated tubing setting from the userinterface, for example in ° C., or an automatic setting; 6) a humidifiersetting from the user interface for example an automatic setting, or a‘wetter’ or ‘dryer’ setting than the standard automatic setting; 7) atime stamp.

The outputs of the control may comprise: 1) a temperature setpoint forthe humidifier; and 2) a temperature setpoint for the heated tubing.

Constants for the control may comprise: 1) coefficients to convert fromrelative to absolute humidity, including a) three coefficients to applyto the quadratic function; and 2) a table to determine a temperaturesetpoint from a desired humidity output of the humidifier.

The table may be a matrix of points from which the setpoint can betrilinearly interpolated, including: a) one axis for average air flowrate, for example corresponding to 10 to 70 L/min at 12 L/min intervals,which provides six points; b) one axis for desired absolute humidityoutput, for example corresponding to 0 to 40 mg/L at 8 mg/L intervals,which provides six points; and c) one axis for ambient absolutehumidity, for example corresponding to 0 to 35 mg/L at 5 mg/L intervals,which provides eight points.

The total matrix size provides 6×6×8=288 data points. Each data point isa temperature from 5 to 95° C. in 0.1° C. increments. The matrix may be,for example, as shown in Table 6 below.

TABLE 6 Ambient Desired Added Hot Plate Flow AH AH Temp 10 0 0 5.0 10 05 5.0 10 0 10 8.3 10 0 15 25.0 10 0 20 41.7 10 0 25 58.3 10 0 30 75.0 100 35 75.0 10 8 0 5.0 10 8 5 5.0 10 8 10 16.3 10 8 15 33.0 10 8 20 49.710 8 25 66.3 10 8 30 75.0 10 8 35 75.0 10 16 0 5.0 10 16 5 7.7 10 16 1024.3 10 16 15 41.0 10 16 20 57.7 10 16 25 74.3 10 16 30 75.0 10 16 3575.0 10 24 0 5.0 10 24 5 15.7 10 24 10 32.3 10 24 15 49.0 10 24 20 65.710 24 25 75.0 10 24 30 75.0 10 24 35 75.0 10 32 0 7.0 10 32 5 23.7 10 3210 40.3 10 32 15 57.0 10 32 20 73.7 10 32 25 75.0 10 32 30 75.0 10 32 3575.0 10 40 0 15.0 10 40 5 31.7 10 40 10 48.3 10 40 15 65.0 10 40 20 75.010 40 25 75.0 10 40 30 75.0 10 40 35 75.0 22 0 0 5.0 22 0 5 5.0 22 0 1020.3 22 0 15 37.0 22 0 20 53.7 22 0 25 70.3 22 0 30 75.0 22 0 35 75.0 228 0 5.0 22 8 5 11.7 22 8 10 28.3 22 8 15 45.0 22 8 20 61.7 22 8 25 75.022 8 30 75.0 22 8 35 75.0 22 16 0 5.0 22 16 5 19.7 22 16 10 36.3 22 1615 53.0 22 16 20 69.7 22 16 25 75.0 22 16 30 75.0 22 16 35 75.0 22 24 011.0 22 24 5 27.7 22 24 10 44.3 22 24 15 61.0 22 24 20 75.0 22 24 2575.0 22 24 30 75.0 22 24 35 75.0 22 32 0 19.0 22 32 5 35.7 22 32 10 52.322 32 15 69.0 22 32 20 75.0 22 32 25 75.0 22 32 30 75.0 22 32 35 75.0 2240 0 27.0 22 40 5 43.7 22 40 10 60.3 22 40 15 75.0 22 40 20 75.0 22 4025 75.0 22 40 30 75.0 22 40 35 75.0 34 0 0 5.0 34 0 5 15.7 34 0 10 32.334 0 15 49.0 34 0 20 65.7 34 0 25 75.0 34 0 30 75.0 34 0 35 75.0 34 8 07.0 34 8 5 23.7 34 8 10 40.3 34 8 15 57.0 34 8 20 73.7 34 8 25 75.0 34 830 75.0 34 8 35 75.0 34 16 0 15.0 34 16 5 31.7 34 16 10 48.3 34 16 1565.0 34 16 20 75.0 34 16 25 75.0 34 16 30 75.0 34 16 35 75.0 34 24 023.0 34 24 5 39.7 34 24 10 56.3 34 24 15 73.0 34 24 20 75.0 34 24 2575.0 34 24 30 75.0 34 24 35 75.0 34 32 0 31.0 34 32 5 47.7 34 32 10 64.334 32 15 75.0 34 32 20 75.0 34 32 25 75.0 34 32 30 75.0 34 32 35 75.0 3440 0 39.0 34 40 5 55.7 34 40 10 72.3 34 40 15 75.0 34 40 20 75.0 34 4025 75.0 34 40 30 75.0 34 40 35 75.0 46 0 0 11.0 46 0 5 27.7 46 0 10 44.346 0 15 61.0 46 0 20 75.0 46 0 25 75.0 46 0 30 75.0 46 0 35 75.0 46 8 019.0 46 8 5 35.7 46 8 10 52.3 46 8 15 69.0 46 8 20 75.0 46 8 25 75.0 468 30 75.0 46 8 35 75.0 46 16 0 27.0 46 16 5 43.7 46 16 10 60.3 46 16 1575.0 46 16 20 75.0 46 16 25 75.0 46 16 30 75.0 46 16 35 75.0 46 24 035.0 46 24 5 51.7 46 24 10 68.3 46 24 15 75.0 46 24 20 75.0 46 24 2575.0 46 24 30 75.0 46 24 35 75.0 46 32 0 43.0 46 32 5 59.7 46 32 10 75.046 32 15 75.0 46 32 20 75.0 46 32 25 75.0 46 32 30 75.0 46 32 35 75.0 4640 0 51.0 46 40 5 67.7 46 40 10 75.0 46 40 15 75.0 46 40 20 75.0 46 4025 75.0 46 40 30 75.0 46 40 35 75.0 58 0 0 23.0 58 0 5 39.7 58 0 10 56.358 0 15 73.0 58 0 20 75.0 58 0 25 75.0 58 0 30 75.0 58 0 35 75.0 58 8 031.0 58 8 5 47.7 58 8 10 64.3 58 8 15 75.0 58 8 20 75.0 58 8 25 75.0 588 30 75.0 58 8 35 75.0 58 16 0 39.0 58 16 5 55.7 58 16 10 72.3 58 16 1575.0 58 16 20 75.0 58 16 25 75.0 58 16 30 75.0 58 16 35 75.0 58 24 047.0 58 24 5 63.7 58 24 10 75.0 58 24 15 75.0 58 24 20 75.0 58 24 2575.0 58 24 30 75.0 58 24 35 75.0 58 32 0 55.0 58 32 5 71.7 58 32 10 75.058 32 15 75.0 58 32 20 75.0 58 32 25 75.0 58 32 30 75.0 58 32 35 75.0 5840 0 63.0 58 40 5 75.0 58 40 10 75.0 58 40 15 75.0 58 40 20 75.0 58 4025 75.0 58 40 30 75.0 58 40 35 75.0 70 0 0 35.0 70 0 5 51.7 70 0 10 68.370 0 15 75.0 70 0 20 75.0 70 0 25 75.0 70 0 30 75.0 70 0 35 75.0 70 8 043.0 70 8 5 59.7 70 8 10 75.0 70 8 15 75.0 70 8 20 75.0 70 8 25 75.0 708 30 75.0 70 8 35 75.0 70 16 0 51.0 70 16 5 67.7 70 16 10 75.0 70 16 1575.0 70 16 20 75.0 70 16 25 75.0 70 16 30 75.0 70 16 35 75.0 70 24 059.0 70 24 5 75.0 70 24 10 75.0 70 24 15 75.0 70 24 20 75.0 70 24 2575.0 70 24 30 75.0 70 24 35 75.0 70 32 0 67.0 70 32 5 75.0 70 32 10 75.070 32 15 75.0 70 32 20 75.0 70 32 25 75.0 70 32 30 75.0 70 32 35 75.0 7040 0 75.0 70 40 5 75.0 70 40 10 75.0 70 40 15 75.0 70 40 20 75.0 70 4025 75.0 70 40 30 75.0 70 40 35 75.0

Internal variables may comprise: 1) absolute humidity of ambient; 2)absolute humidity to target at mask; 3) absolute humidity to be added byhumidifier; and 4) previous flow rates measured.

In order to generate the temperature setpoint for the humidifier, thecontroller(s) 40 and/or 44:

1. Calculate ambient absolute humidity from ambient relative humidityand temperature according to : absolute humidity =relative humidity (asa proportion of 1)×(a+b×temp+c×temp×temp) given constant coefficientsa=7.264, b=0.0928 and c=0.0293.

2. Calculate target absolute humidity from temperature sensed by heatedtubing. If the heated tubing is not available, the ambient temperaturemay be applied instead. The function is the same quadratic as used instep 1, but the relative humidity is now set by the user interface.

3. Calculate absolute humidity to be added by humidifier by subtractingthe ambient absolute humidity from the target.

4. Calculate the temperature setpoint for humidifier from the absolutehumidity to be added, the flow rate and the ambient temperature. Thecalculation is a trilinear interpolation of Table 6.

In order to generate the temperature setpoint for the heated tubing:

1. A default temperature setpoint corresponds to the setting on the userinterface.

2. If there has been a sudden fall in flow rate, the temperaturesetpoint is adjusted slightly (e.g. a few ° C.) above the setpoint for alimited duration (e.g. 15 min).

Flow Generator Design Considerations

When the humidifier is fitted to or removed from the flow generator, theflow generator user interface may indicate detection or removal of thehumidifier within, for example, one second. When a heated tubing isfitted to or disconnected from the humidifier, the flow generator userinterface may indicate detection or removal of the heated tubing within,for example, one second.

As discussed above, the flow generator controller may control thehumidifier and the heated tubing. The flow generator controller may useconstants stored in the humidifier controller and comprising, forexample, six control parameters, each a value between 0 and 1 with 0.01resolution and a matrix of 6×6×8=288 data points. Each data point may bea temperature from 5 to 95° C. with 0.1° C. resolution.

During therapy the flow generator may poll the humidifier for thereadings of humidifier heating element and the heated tubingtemperature, for example, at least once every 10 seconds. During therapythe flow generator may poll the humidifier for the readings of ambienttemperature and relative humidity, for example, at least once every 60seconds.

Temperatures may be communicated as values from 5 to 95° C. with 0.1° C.resolution. Relative humidity may be communicated as an integer valuefrom 0 to 100. Values outside this range shall be limited to this range.

The flow generator may calculate the duty ratio to be applied by thehumidifier as an integer value between 0 and 100 (where 100 indicates100% duty). The flow generator may also calculate the duty ratio to beapplied to the heated tubing as an integer value between 0 and 100(where 100 indicates 100% duty). The flow generator may ensure that thesum of the duty ratios for the humidifier and heated tubing does notexceed 100 (indicates 100%).

During therapy, requests from the flow generator to set the humidifierduty ratio may be transmitted, for example, at least once every 3seconds and requests from the flow generator to set the heated tubingduty ratio may be transmitted, for example, at least once every 1second.

Humidifier Design Considerations

When both the heated tubing and the humidifier are commanded to heat,the controller(s) 40 and/or 44 may ensure that the power is distributedsuch that both items are not drawing power at the same instant. Toachieve this, the heated tubing and the humidifier may be controlled bythe same controller.

A suitable communications protocol may be developed to enable the flowgenerator to communicate with the humidifier and the power supply andany other devices that may be added. The communications protocol mayutilize, for example, a 16-bit CRC to detect communications errors. Thecommunications between the flow generator and the humidifier may behalf-duplex to minimize the number of pins in the wiring connectors.

The humidifier may transmit the following information to the FG ondemand: 1) humidifier status (ok or error); 2) relative humidityreading; 3) temperature at which the relative humidity reading was made;4) temperature of the heating element in the humidifier; 5) humidifierheating duty ratio.

The humidifier may respond to the following commands from the flowgenerator: 1) request for humidifier status; 2) request for humidityreading; 3) request for temperature of humidity reading; 4) request fortemperature of the heating element in the humidifier; 5) set the heatingduty ratio in the humidifier.

The humidifier may cease heating the humidifier tub unless a requestsetting the heating duty ratio is received at least every 10 seconds.

Heated Tube Design Considerations

The humidifier may transmit the following information to the flowgenerator on demand: 1) heater tube status, including a) the presence orabsence of a heated tubing, b) the diameter of the heater tube (15 mm or19 mm), and c) ok or error; 2) temperature in the heater tube; and 3)humidifier heating duty ratio.

The humidifier may respond to the following commands from the flowgenerator: 1) request for heater tube status; 2) request for temperaturein the heater tube; 3) set the heating power level in the humidifier.

The humidifier may cease heating the heater tube unless a requestsetting the heating duty ratio is received, for example, at least every1 second.

Temperature Conversions

The controller(s) 40 and/or 44 may convert the potential measured acrossa thermistor into a temperature, for example in ° C., using a lookuptable. Three tables are required: 1) and 2) a temperature conversiontable for each type of heated tubing (e.g. 15 mm and 19 mm) (at 0.1° C.resolution for range 5 to 40° C., having approximately 360 data pointsin each of two tables); and 3) a temperature conversion table for thehumidifier (at 0.1° C. resolution for range 5 to 95° C., havingapproximately 960 data points). Each may be a lookup table indexed bybeing evenly spaced on the axis of thermistor potential.

Upload of Climate Control Constants to Flow Generator

The humidifier may carry a table, for example Table 6, as a constant andtransfer it to the flow generator before climate control begins. This isso that humidifier upgrades may be implemented in the humidifier withoutthe need to upgrade the flow generator software.

Indicator Lights

On command from the flow generator, for example using commands over aserial communications link, the humidifier may directly control one blueand one amber LED. The humidifier may control the indicator lightsaccording to commands received from the flow generator and each commandmay include the following information: 1) color—blue or amber; 2)brightness—bright, dim or off; and 3) fading—yes or no.

If fading is: 1) yes, the brightness shall transition smoothly overthree seconds; or 2) no, the brightness shall switch to the new level.The humidifier may be able to fade changes on both indicatorssimultaneously, e.g. for a crossfade the flow generator may send twocommands together—one command to fade one indicator off and a secondcommand to fade the other indicator on.

Humidifier Control Sixth Embodiment

Patients sleeping with a humidifier set to deliver humidity belowsaturation in the tubing may suffer from condensation in the tubing inthree circumstances: 1) a drop in ambient temperature, so the air coolsin the tubing to below its dew point; 2) a rise in ambient humidity, sothe air leaving the humidifier rises in humidity and then cools in thetubing to below its dew point; and 3) a drop in flow rate, such as whenautosetting lowers the treatment pressure, so the humidifier adds morehumidity to the air, and then the air cools in the tubing to below itsdew point.

The advice currently given to patients to deal with the problem ofcondensation, or rain out, in the tube includes running the tubing underthe bedclothes to reduce cooling in the tubing and/or setting thehumidifier to a lower heat setting. These approaches result in thepatient receiving less humidity all night to provide more margin fromthe dew point, to allow for changes during the night.

As discussed above, sample embodiments provide implementation of climatecontrol to deliver a predetermined temperature and humidity of air tothe mask end of the tubing. However, climate control as described withreference to the previous sample embodiments requires a temperaturesensor in the tubing to monitor the temperature of the air in thetubing. There is increased cost in the heated tubing with temperaturesensing, so it would be an advantage to offer a patient some relief fromcondensation in a system with conventional, i.e. unheated, tubing.

Referring to FIG. 18, according to another sample embodiment, climatecontrol is provided in which the tubing is not heated and the deliveredair temperature is not measured, but rather is estimated in S22 from thereadings of the ambient temperature sensor. The estimate is based oncharacterisation of the temperature differences between the ambienttemperature reported and the delivered air temperature under differentconditions of ambient temperature and air flow and sources of heat inthe device such as a power supply, a motor, electronics, or thehumidifier heating element.

As discussed above, Comparative Example 1 (Table 2) shows the responsethat the sample embodiments discussed above with respect to FIGS. 15 and16 would have to a change in ambient temperature with no change inambient absolute humidity and no control of the mask temperature Tm.According to this sample embodiment in which the delivered airtemperature Tm is not measured, but rather estimated, an equivalenttable for the adjustment in water temperature to a change in ambienthumidity is shown below in Table 7 for three different options ofambient air temperature.

TABLE 7 Ambient air Temp Humidity Humidity Water Ambient air absoluteAir of air of air of air temperature temperature humidity relativeHumidification delivered delivered delivered degC. degC. (mg/L) humidityoutput mg/L degC. (mg/L) % RH 48.6 15 4 32% 8.5 15 12.5 100% 46 15 6 48%6.5 15 12.5 100% 42.5 15 8 64% 4.4 15 12.4 100% 38.3 15 10 80% 2.4 1512.4 100% 31 15 12 96% 0.4 15 12.4 100% 50.5 20 8 47% 9.1 20 17.1 100%47.8 20 10 58% 7.2 20 17.2 100% 44.5 20 12 70% 5.2 20 17.2 100% 40 20 1482% 3.1 20 17.1 100% 34 20 16 93% 1.2 20 17.2 100% 51.8 25 14 60% 9.3 2523.3 100% 49 25 16 69% 7.3 25 23.3 100% 45.5 25 18 77% 5.2 25 23.2 100%41.5 25 20 86% 3.3 25 23.3 100% 35 25 22 95% 1.2 25 23.2 100%

A feature of this sample embodiment is that the delivered airtemperature is estimated such that the device does not detect whetherthe tubing is insulated from ambient temperature, such as with a clothcover or bedding. Insulation can increase the delivered air temperatureby reducing cooling in the tubing. To minimise the chance ofcondensation the tubing may be assumed to have no insulation andtherefore the delivered air is cooler and closer to its dew point thanif insulation is fitted.

It should be appreciated that the system of this sample embodiment willrespond appropriately to simultaneous changes in ambient temperature andambient humidity and air flow rate. The system of this sample embodimentprovides protection against condensation in the tubing throughout thenight, regardless of changes in ambient temperature, humidity and flow.The system of this sample embodiment also offers completely automaticcontrol of the humidifier. Assuming a default value for thepredetermined relative humidity of the delivered gas, the patient neednever adjust the humidifier. The system of this sample embodiment alsooffers a setting of humidity through the user interface, which can betranslated into a predetermined relative humidity of the delivered gas.

Unlike the other sample embodiments which include heated tubing, thissample embodiment cannot deliver warmer air, or the higher humidity thatcan be carried by warmer air. This sample embodiment also does not allowthe patient to select the temperature of air delivered. This sampleembodiment also does not raise the humidity delivered if the tubing isinsulated. Changing the setting for humidity through the user interfacecould overcome this.

The humidifier control according to this sample embodiment allows therespiratory apparatus to be provided with standard tubing rather thanheated tubing, thus reducing the cost of the system.

The sample embodiments discussed above may also be implemented entirelyin software or hardware (e.g. an ASIC), so the humidifier may beconfigured to operate as any of the three sample embodiments with noincrease in the device cost of goods.

The humidifier according to the sample embodiments disclosed hereinimprove user compliance due to increased comfort, reduced likelihood ofdry/sore throat, and/or improved ease of use from the provision of anautomatic optimum humidification setting.

The humidifier according to the sample embodiments disclosed herein alsoprovides a solution to a problem found in prior art humidifiers thatonly track room ambient temperature and flow, which is that suchhumidifiers may be tracking an inappropriate humidification output dueto human error/confusion in making the original setting. A user of suchhumidifiers does not know which setting is closest to the optimumhumidification level for any given condition, particularly whenever theyexperience a significant change to their usual environment/climate, e.g.during travel.

The humidifiers and respiratory apparatus according to the sampleembodiments disclosed herein measure ambient relative humidity andpressure (altitude compensation), as well as ambient temperature, toimprove accuracy in the delivered humidification level compared to priorart systems that do not sense ambient humidity and pressure. Theavailability of low cost humidity and pressure sensors in recent yearsnow makes monitoring of these additional parameters feasible andpractical even in CPAP devices.

The humidifier and respiratory apparatus according to the sampleembodiments disclosed herein will respond to detection of sustainedmouth leak, but unlike prior art systems, will correct thehumidification output to the optimum moisture density, rather than justto an arbitrary setting which is likely not close to optimum.

While the invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the invention. Also, the various embodiments described abovemay be implemented in conjunction with other embodiments, e.g., aspectsof one embodiment may be combined with aspects of another embodiment torealize yet other embodiments. Further, each independent feature orcomponent of any given assembly may constitute an additional embodiment.Furthermore, each individual component of any given assembly, one ormore portions of an individual component of any given assembly, andvarious combinations of components from one or more embodiments mayinclude one or more ornamental design features. In addition, while theinvention has particular application to patients who suffer from OSA, itis to be appreciated that patients who suffer from other illnesses(e.g., congestive heart failure, diabetes, morbid obesity, stroke,barriatric surgery, etc.) can derive benefit from the above teachings.Moreover, the above teachings have applicability with patients andnon-patients alike in non-medical applications.

In this specification, the word “comprising” is to be understood in its“open” sense, that is, in the sense of “including”, and thus not limitedto its “closed” sense, that is the sense of “consisting only of”. Acorresponding meaning is to be attributed to the corresponding words“comprise,” “comprised” and “comprises” where they appear.

It will further be understood that any reference herein to known priorart does not, unless the contrary indication appears, constitute anadmission that such prior art is commonly known by those skilled in theart to which the invention relates.

What is claimed is:
 1. A humidifier for a respiratory apparatus fordelivering a humidified flow of breathable gas to a patient, thehumidifier comprising: a humidifier chamber configured to store a supplyof water to humidify the flow of breathable gas, the humidifier chambercomprising a first heating element configured to heat the supply ofwater; a relative humidity sensor to detect a relative humidity ofambient air and generate signals indicative of the ambient relativehumidity; a first temperature sensor to detect a temperature of ambientair and generate signals indicative of the ambient temperature; and acontroller configured to determine an absolute humidity of the ambientair from the signals generated by the relative humidity sensor and thefirst temperature sensor and to control the first heating element toprovide a predetermined relative humidity to the flow of breathable gas.2. A humidifier according to claim 1, wherein the controller is furtherconfigured to control the first heating element to provide the flow ofbreathable gas at a predetermined temperature.
 3. A humidifier accordingto claim 2, wherein the controller determines a predetermined absolutehumidity corresponding to the predetermined relative humidity and thepredetermined temperature.
 4. A humidifier according to claim 3, whereinthe predetermined absolute humidity is about 11-44 mg/L.
 5. A humidifieraccording to claim 2, wherein the predetermined temperature is about 15°C.-37° C.
 6. A humidifier according to claim 2, wherein thepredetermined temperature is about 26°-28° C.
 7. A humidifier accordingto claim 1, wherein the predetermined relative humidity is about50%-100%.
 8. A humidifier according to claim 1, wherein thepredetermined relative humidity is about 70%-90%.
 9. A humidifieraccording to claim 1, further comprising a user input configured topermit the patient or a clinician to select the predeterminedtemperature and/or the predetermined relative humidity and/or thepredetermined absolute humidity.
 10. A humidifier according to claim 9,wherein the user input comprises a control knob.
 11. A humidifieraccording to claim 1, further comprising a second temperature sensor todetect a temperature of the supply of water or a temperature of thefirst heating element and generate signals indicative of the temperatureof the supply of water or the temperature of the first heating element,respectively, wherein the controller is configured to control the firstheating element in a feedback loop of the signals generated by thesecond temperature sensor.
 12. A humidifier according to claim 3,wherein the controller calculates an evaporation rate of the supply ofwater to provide the predetermined absolute humidity.
 13. A humidifieraccording to claim 12, wherein the evaporation rate is determined bymultiplying a flow rate of the flow of breathable gas and a differencebetween the predetermined absolute humidity and the ambient absolutehumidity.
 14. A humidifier according to claim 3, wherein the controllercalculates the ambient absolute humidity according to the formulaAHa=RHa·(K1'1K2·Ta+K3·Ta2), wherein AHa is the ambient absolutehumidity, RHa is the ambient relative humidity and Ta is the ambienttemperature, and calculates the predetermined absolute humidityaccording to the formula AHp=RHp·(K1−K2·Tp+K3·Tp2), wherein AHp is thepredetermined absolute humidity, RHp is the predetermined relativehumidity, Tp is the predetermined temperature, and K 1, K 2, and K3 arecoefficients.
 15. A humidifier according to claim 13, further comprisinga flow rate sensor configured to determine the flow rate and generatesignals indicative of the flow rate.
 16. A humidifier according to claim13, wherein the flow rate corresponds to a vent flow from the patientinterface at a pressure of the flow of breathable gas.
 17. A humidifieraccording to claim 1, wherein the controller is configured to controlthe first heating element to increase the temperature of the supply ofwater as the flow rate increases and decrease temperature of the supplyof water as the flow rate decreases.
 18. A humidifier according to claim1, wherein the controller is configured to control the first heatingelement to decrease the temperature of the supply of water as theambient absolute humidity increases, and to increase the temperature ofthe supply of water as the ambient absolute humidity decreases.
 19. Ahumidifier according to claim 1, wherein the controller is configured tocontrol the first heating element to increase the temperature of thesupply of water as the predetermined temperature increases, and todecrease the temperature of the supply of water as the predeterminedtemperature decreases.
 20. A humidifier according to claim 1, wherein aplurality of predetermined temperatures, a plurality of predeterminedrelative humidities, and a plurality of corresponding predeterminedabsolute humidities are stored in memory operatively associated with thecontroller.